Probing the Birth of Super Star Clusters

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Probing the Birth of Super Star Clusters

• Kelsey Johnson
• University of Virginia

IAU227, 2005
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A fossil inthe Milky Way...

• gt 10 billion years old
• a few parsecs in size
• 104 - 106 stars

How were these incredible objects formed?
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What Makes Super Star Clusters Interesting?

• Extreme mode of star formation
• Plausibly proto-globular clusters
• Formation common in early universe
• Luminous simple stellar populations for
  probing galaxy evolution
• Impact on the ISM IGM

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On the Future of Super Star Cluster
Research (without HST)
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Can we learn from Galactic Star Forming
Regions? From Ultracompact HII Regions to Proto
Globular Clusters
Key Questions How do the properties depend on
environment? How do the properties of star
formation scale between these regimes?
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Strategy Look for sources with similar SEDs to
Ultracompact HII regions
Wood Churchwell 1989
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Comparison of Radio SEDs (individual clusters)
SBS 0335-052
He 2-10
NGC 5253
W49A
Johnson et al. in prep, Johnson Kobulnicky
2003, Mezger et al. 1967, Turner et al. 1998, 2004
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Comparison of Radio SEDs (individual clusters)

• Radii of HII regions lt a few pc
• Electron densities gt104 - 106 cm-3
• ? Pressures gt 108 kB
• Ionizing Luminosities gt 1052-53 s-1
• ? gt 1000s O7-type stars

SBS 0335-052
He 2-10
NGC 5253
W49A
Johnson et al. in prep, Johnson Kobulnicky
2003, Mezger et al. 1967, Turner et al. 1998, 2004
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Revisiting He 2-10 (again) New Radio
Observations
HST I-band
Kobulnicky Johnson 1999, Johnson et al. 2000,
Johnson Kobulnicky 2003, Biswas Johnson in
prep
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SBS 0335-052 ultra-low metallicity (Z ? 1/40
Z?)
Johnson Plante in prep.
Color scale HST NICMOS Paa Contours VLA Pie
Town X-band
Color scale HST ACS F140LP Contours VLA Pie
Town X-band
Massive proto-cluster detected in mid-IR Av gt 15
-30 AND similar embedded stellar mass (Hunt,
Vanzi, Thuan, 2001 Plante Sauvage, 2002)
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Probing the Dust Cocoons of Natal Clusters
He 2-10
The radio sources alone account for at least 60
of the mid-IR flux from the entire galaxy
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Probing the Dust Cocoons of Natal Clusters
Haro 3
Radio clusters also have an infrared
excess Hot dust near the ionizing stars
Color scale HST V-band Contours VLA X-band
Johnson, Indebetouw, Watson, Kobulnicky 2004
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3D Monte-Carlo Radiative Transfer (à la Barb
Whitney)
Modeling the Evolution of Super Star Clusters

• Enables dust structure consistent with the
  actual ISM
• Enables multiple sources

Johnson, Whitney, Indebetouw Wood in prep.
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Geometric Sequence with Rin increasing (pseudo-evo
lutionary sequence)
Example 90 clumpy, Rout50pc, SFE10
Johnson, Whitney, Indebetouw in prep.
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3D Monte-Carlo Radiative Transfer Super Star
Clusters
Geometric Sequence (pseudo evolution)

• Bad News
• Significant degeneracies (both between model
  parameters and viewing angles)
• Observed bolometric flux can be off by factor
  of 2
• Good News
• Near-IR bands span important transition in SED
• Far-IR l are robust (and 24mm is not too bad)

• Model Evolution of SED
• SFE 10, Rout25pc

Smooth 100 90 50 10 1
Johnson, Whitney, Indebetouw, Wood in prep
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WARNING!
WARNING!
WARNING!
Assuming that dust cocoons are smooth can lead to
vastly misinterpreting Spitzer data. Proceed
with caution!
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Comparison to UCHII Colors

• Model Colors of SSCs agree well with UCHIIs

• Models extend blueward
• Phase with extremely hot dust likely very short

• Models extend redward
• Phases with cool dust may not be detected as
  UCHIIs

Johnson, Whitney, Indebetouw, Wood, in
prep Wood Churchwell, 1989 Kurtz, Churchwell,
Wood 1994
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• To Do List
• Directly measure densities, pressures,
  temperatures (use IR forbidden lines,
  molecular lines, RRLs)
• Directly measure radii with high resolution
  (EVLA II?, SKA at some point?)
• Determine how much ionizing radiation escapes
  (need bolometric luminosities, clumpiness)
• Determine star formation efficiency (high
  resolution HI, CO, H2)
• Find out if the individual stars have
  individual cocoons? (dependence on the
  evolutionary state?)
• Determine how clumpy the dust is
  (high-resolution imaging and SED models)
• Determine the temperature profiles
  (high-resolution photometry and SED models)

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Looking toward the Future (IR - mm)
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Implications for Massive Star Formation

• How do the processes of star formation vary
  between small associations and massive clusters?
• e.g. Environmental requirements? Protostellar
  interactions?
• How do the properties of star formation vary
  between small associations and massive clusters?
• e.g. Star formation efficiency? Stellar IMF?

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Summary

• Super Star Clusters are an important mode of star
  formation (plausibly proto globular clusters!)
• We have a sample of natal clusters that appear to
  have properties similar to Galactic UCHII Regions
• Thermal IR SEDs can be significantly affected by
  clumping
• There is a lot to learn about these objects, and
  the new generation of telescopes will provide
  powerful diagnostics
• The future is extremely bright for this type of
  research

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1) Primary Formation Models(e.g. Peebles
Dicke 1968)
globular clusters first bound structures in the
universe
we suggest that globular clusters formed (as
gas clouds) before the galaxies appeared, by a
universal process independent of the peculiar
conditions in which globular clusters now find
themselves. - Peebles Dicke 1968

• Some problems with this idea
• Populations of metal-rich globular clusters
  (e.g. Zinn 1985)
• Correlation between metallicity and location in
  galaxies (e.g. Harris 1991)
• Absence of dark matter halos (e.g. Moore 1996)

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2) Pressure Formation Models(e.g. Elmegreen
2002)
SSCs can only form in extremely high pressure
environments

• This theory suggests that we look for galactic
  scale high pressure events and places
• Kpc-scale instabilities turbulence
• Galaxy interactions
• Galactic nuclear regions
• Hard to disentangle statistics and physics

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Geometric Sequence with Rin increasing (pseudo-evo
lutionary sequence)
Johnson, Whitney, Indebetouw in prep.
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Observational strategyIf we want to understand
cluster formation, its not a bad idea to observe
them while they are forming.
Problem Once clusters are fully visible in
optical light, their birth environments have been
dramatically altered
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Geometric Sequence with Rin increasing (pseudo-evo
lutionary sequence)
Example 90 clumpy, Rout50pc, SFE10
Johnson, Whitney, Indebetouw in prep.